A mold clamping device of a double platen-type injection molding machine is provided with two elements, one being a mold clamping mechanism and the other being a mold opening/closing mechanism. Usually, mold closing is performed by a movable plate at a position that is away from a fixation plate being moved toward the fixation plate by the mold opening/closing mechanism, and then the mold clamping mechanism performs mold clamping at a predetermined pressure on the fixation plate and the movable plate mechanically coupled with the fixation plate. Once one shot of injection molding is over, the mold clamping by the mold clamping mechanism is released, and then mold opening is performed by the mold opening/closing mechanism moving the movable plate away from the fixation plate.
PTL 1 discloses a mold clamping device that is provided with a mold opening/closing mechanism which is provided with a mold clamping mechanism performing mold clamping after mold closing by allowing a fixation plate and a movable plate to be coupled with each other and a pair of ball screws driven by a pair of motors in a double platen-type injection molding machine including the fixation plate and the movable plate. In the mold opening/closing mechanism according to PTL 1, both ends of a screw shaft of the ball screw are rotatably supported by a cradle and a ball screw nut is fixed to the movable plate. In other words, the ball screw shaft in the mold opening/closing mechanism according to PTL 1 is supported at three points, including the ball screw nut.
In the mold opening/closing mechanism according to PTL 1, the both ends of the screw shaft of the ball screw are supported by the cradle and each of the ball screws is installed at a low position of the movable plate. Accordingly, the point of driving by the ball screw is a low position of the movable plate. Accordingly, during mold opening/closing movements, the high site of the movable plate is subject to a delay in movement in comparison to the low site due to inertia and the accuracy of the degree of parallelism between the fixation plate and the movable plate falls in some cases. In addition, the driving resultant force of each of the ball screws of the mold opening/closing mechanism does not pass through the center of gravity of the movable plate or the vicinity of the center of gravity, and thus a large-capacity and expensive ball screw has to be applied in some cases for the ball screw to be capable of enduring the bending moment generated in a drive shaft of the ball screw due to the inertial force of the movable plate.
As an example of countermeasures related thereto, one of the pair of ball screws may be disposed at a high position with the other one of the ball screw disposed at a low position across the center of gravity of the movable plate or the vicinity of the center of gravity. At least the ball screw that is disposed at the high position is away from the cradle, and thus a structure for supporting both ends is unlikely to be obtained, even if one supporting end of the ball screw is fixed to the movable plate or the fixation plate, due to the lack of a member fixing the other supporting end in the case of the double-platen structure mold clamping device as in PTL 1. This results in the adoption of a supporting structure in which only one end of the ball screw shaft is supported by the movable plate or the fixation plate and the other end is a free end. The ball screw that is disposed at the high position is weak in terms of supporting rigidity because the degree of freedom of an attachment space is low and it is an attachment structure at the high position, and thus reduction in size and weight is required. A supporting structure in which one end of a screw shaft of a mold opening/closing mechanism is supported by a fixation plate and the other end is a free end is disclosed in, for example, PTL 2. The supporting structure according to PTL 2 relates to a three platen-type mold clamping device, and the side of the screw shaft that is close to the free end is supported by a nut member fixed to a connection board. In other words, the supporting structure in which one end of the screw shaft is supported and the other end is a free end is supported at two points, including the nut member. This supporting structure will be referred to as a cantilever supporting structure below.
In the case of the cantilever supporting structure that is disclosed in PTL 2, the slenderness ratio of the ball screw shaft increases because the length from the one end portion of the ball screw shaft that is supported to the nut member is large or in a case where the diameter of the ball screw shaft is small for reduction in weight. For this reason, the ball screw shaft is likely to swing, and a risky speed is reached in some cases depending on operation conditions. Accordingly, in some cases, the degree of freedom of the upper allowable value of the moving speed of the movable plate by the ball screw is limited.